The invention relates to internal combustion engines and, more particularly, to exhaust systems for internal combustion engines.
In general, it is known to utilize returning pressure waves to improve scavenging, thereby to increase the amount of fuel mixture in the next charge and thereby to boost horsepower. It has also been known to apply a compressive pulse from an exhaust gas system to a cylinder just prior to closing of the exhaust port so as to raise the pressure of the fuel mixture in the cylinder and thus effect a "supercharge" and/or to effectively stop the flow of fresh charge from the exhaust port and thus reduce loss of fuel.
Pressure wave travel or acoustical flow, as these terms are employed in this application, takes place at or near the speed of sound and is primarily affected by the static stream temperature of the exhaust gas, i.e., the static temperature of the surrounding environment. Pressure wave travel is essentially independent of the cross sectional area or configuration of the path through which the flow occurs, except for generation of return pulses, either rarefactive or compressive, in response to changes in cross sectional flow area. The mass flow of the exhaust product is dependent, at least in part, on the configuration of the exhaust gas path and upon the pressure differential associated with the path. In exhaust gas flow systems for two stroke engines, the mass flow of the exhaust product is comparatively slow as compared to the acoustical flow and, thus, the acoustical flow is usually not substantially affected by changes in configuration of the exhaust gas path or by pressure differential associated with the path. Accordingly, acoustical flow or pressure wave travel in paths of equal length will generally have the same characteristics, as for instance, the time for flow through one path will be substantially the same as the time for flow through any path of equal length.
U.S. Pat. No. 3,692,006, Miller, et al. issued Sept. 19, 1972, discloses an internal combustion engine having three or more cylinders and exhaust gas discharge means communicating with the exhaust ports of the cylinders for establishing substantially equal acoustical flow distances between each of the exhaust ports and the exhaust port of the subsequently charged cylinder, the acoustical flow distances being such that the compressive wave emanating from the opening of the exhaust port of the immediately previously fired cylinder arrives at the exhaust port of the then being charged cylinder at a time prior to closure of the exhaust port of the cylinder then being charged. In other words, each cylinder is "tuned" by a compressive wave from the immediately previously fired cylinder.
Prior art systems such as the system of U.S. Pat. No. 3,692,006 are generally optimized for high engine speeds and are mistuned at low engine speeds. That is, the acoustical flow distances are such that the compressive wave emanating from the opening of the immediately previously fired cylinder arrives at the exhaust port of the then being charged cylinder a little too early at low engine speeds. This is because at low engine speeds the time interval between the opening of exhaust ports is greater than at high engine speeds. Stated alternatively, the acoustical flow distances are too short for low engine speeds.
Japanese Pat. No. 55-112823 discloses a single cylinder, two stroke engine having an exhaust system wherein a reflected wave or pulse tunes the engine. The exhaust system includes a valve for changing the length of the exhaust passage to maximize both high and low speed performance.
U.S. Pat. No. 1,804,321, Crowe issued May 5, 1931, discloses a variable length vacuum chamber connected to the exhaust passage of an internal combustion engine. The purpose of the Crowe vacuum chamber is to prevent returning exhaust gases from reaching the exhaust port of the engine.
Attention is also directed to the following U.S. patents:
Bourne U.S. Pat. No. 1,860,569, issued May 31, 1932;
Maybach et al. U.S. Pat. No. 2,717,583, issued Sept. 13, 1955;
Kopper U.S. Pat. No. 3,254,484, issued June 7, 1966;
Trisler U.S. Pat. No. 2,862,490, issued Dec. 2, 1958;
Tenney U.S. Pat. No. 3,703,937, issued Nov. 28, 1972;
Raczuk U.S. Pat. No. 3,726,092, issued Apr. 10, 1973; and
Lampheer U.S. Pat. No. 3,808,807, issued May 7, 1974.